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A damp ceiling or water dripping down though the attic ladder could be the indication of a clogged AC (air conditioner) drain line. These drain lines carry water that drips down from the condenser part of your AC system to the outside for evaporation. Eventually these lines can become clogged, which frequently happens during summer months. One of the methods you can use to clear these lines is with air. An air compressor is the best method to achieve this.
Kill the power for the air conditioner where you will be blowing air through the drain line. You can find the cut-off switch for the AC unit in the main circuit breaker. Move the switch to the off setting. Place a piece of tape over the switch so it isn't turned on while you are working. Some AC units may have a separate breaker box installed somewhere close to the condenser unit, sometimes located on the outside of the house.
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Locate the evaporator unit in your residence, which is where the drain pan is installed. This could be in an attic or basement. The evaporator will be connected by pipes to the condenser unit on the outside of your house. They are typically box-shaped units and will be installed next to the fan for your HVAC system.
Find the location of the drain pipe in the drain pan. The pipe is usually connected to the side of the pan and then runs outside to drain the condensate. If the pan is filled with water, run your hand around the perimeter until you feel the drain opening.
Bail out the water in the drain pan, if needed. This is especially important if you plan to be using an air line that can't be used underwater. Use a plastic tumbler to scoop up as much water as possible, then mop up the rest with a sponge.
Insert the end of the air hose into the drain pipe opening.
Switch on the hose. Blow the air through the line until the clog has cleared or the line is running smoothly.
The above steps are just a guideline. Follow the specific instructions for using your air compressor or other method for blowing air through the pipe.